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Re: [RFH] PR middle-end/179 gcc -O2 -Wuninitialized missing warning with &var
On Fri, Aug 15, 2008 at 8:06 AM, Manuel López-Ibáñez
<lopezibanez@gmail.com> wrote:
> 2008/8/14 Daniel Berlin <dberlin@dberlin.org>:
>> 1. You can't assume VUSE's are must-aliases. The fact that there is a
>> vuse for something does not imply it is must-used, it implies it is
>> may-used.
>>
>> We do not differentiate may-use from must-use in our alias system. You
>> can do some trivial must-use analysis if you like (by computing
>> cardinality of points-to set as either single or multiple and
>> propagating/meeting it in the right place).
>>
>> Must-use is actually quite rare.
>
> Then, is it impossible to distinguish the following testcase and the
> one from my previous mail with the current infrastructure?
If by current you mean "code that already exists", then yes :)
You could write code to do further analysis, but with the existing
code, it will not work.
>
>> 2. " if (!gimple_references_memory_p (def))
>> + return;
>> +"
>> Is nonsensical the SSA_NAME_DEF_STMT of a vuse must contain a vdef,
>> and thus must access memory.
>
> Two things here.
>
> 1) The case I am trying to war about is:
>
> # BLOCK 2 freq:10000
> # PRED: ENTRY [100.0%] (fallthru,exec)
> [/home/manuel/src/trunk/gcc/testsuite/gcc.dg/uninit-B.c : 12] # VUSE
> <iD.1951_4(D)> { iD.1951 }
> i.0D.1952_1 = iD.1951;
> [/home/manuel/src/trunk/gcc/testsuite/gcc.dg/uninit-B.c : 12] if
> (i.0D.1952_1 != 0)
>
> The def_stmt of i.0 is precisely that one. There is no vdef there.
Sure but this is a default def, which are special, and do nothing anyway.
>
> 2) I use that test to return early if the def_stmt of "t" does not
> reference memory. t is just a SSA_NAME (like i.0 above), I do not know
> whether its def_stmt has a VUSE like the above or not. I guess the
> test is redundant since SINGLE_SSA_USE_OPERAND will return NULL
> anyway. Is that what yo mean?
>
No, i mean any SSA_NAME_DEF_STMT for a vuse that is not a default_def
will reference memory.